There are a variety of encoding methods known for encoding an audio signal, inclusive of a speech signal and an acoustic signal, by exploiting statistical properties of the audio signal in the time domain and in the frequency domain and the psychoacoustic characteristics of the human hearing system. These encoding methods may be roughly classified into encoding on the time domain, encoding on the frequency domain and analysis/synthesis encoding.
If, in multi-band excitation (MBE), single-band excitation (SBE), harmonic excitation, sub-band coding (SBC), linear predictive coding (LPC), discrete cosine transform (DCT), modified DCT (MDCT) or fast Fourier transform (FFT), as examples of high-efficiency coding for speech signals, various information data, such as spectral amplitudes or parameters thereof, such as LSP parameters, .alpha.-parameters or k-parameters, are quantized, scalar quantization has been usually adopted.
If, with such scalar quantization, the bit rate is decreased to e.g. 3 to 4 kbps to further increase the quantization efficiency, the quantization noise or distortion is increased, thus raising difficulties in practical utilization. Thus it is currently practiced to group different data given for encoding, such as time-domain data, frequency-domain data or filter coefficient data, into a vector, or to group such vectors across plural frames, into a matrix, and to effect vector or matrix quantization, in place of individually quantizing the different kinds of data.
For example, in code excitation linear prediction (CELP) encoding, LPC residuals are directly quantized by vector or matrix quantization as time-domain waveform. In addition, the spectral envelope in MBE encoding is similarly quantized by vector or matrix quantization.
If the bit rate is decreased further, it becomes infeasible to use enough bits to quantize parameters specifying the envelope of the spectrum itself or the LPC residuals, thus deteriorating the signal quality.
In view of the foregoing, it is an object of the present invention to provide a speech encoding method capable of affording satisfactory quantization characteristics even with a smaller number of bits.